This invention relates to a method of controlling supercharging pressure in an internal combustion engine equipped with a supercharger, and more particularly to an improved method of properly controlling the supercharging pressure when the transmission is in a predetermined position, or when the engine is in particular operating conditions.
A supercharging pressure control method for internal combustion engines has been proposed by the assignee of the present application, e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-129126, in which when the engine is in a transient operating condition in which the supercharging pressure abruptly increases, open loop control is executed, while when the engine is in a steady operating condition in which the supercharging pressure is relatively stable, feedback control is executed, whereby hunting in the supercharging pressure is prevented from taking place due to a time lag in the responsiveness of the control system to the actual change in the supercharging pressure which would take place if the feedback control were executed during the above transient operating condition of the engine, to thereby effect smooth control of the supercharging pressure.
However, the above proposed control method still has room for further improvment in respect of the accelerability of the engine and the controllability of the supercharging pressure as well as the durability or life of the engine when the transmission is in a predetermined gear position or in particular operating conditions.
First, torque acting upon component parts of the transmission of the engine varies depending on the gear position assumed by the transmission. The torque acting upon the component parts is very large when the transmission is in the first speed position. Generally, the component parts should be designed so as to endure such large torque. However, since, as mentioned above, the torque is very large when the transmission is in the first speed position, if the supercharging pressure is controlled at a constant value irrespective of the gear position of the transmission, the transmission will have to be large in size.
Further, when the rotational speed of the engine temporarily drops at the time of shifting the gear position of the transmission, the supercharging pressure can overshoot due to a time lag in the operation of the supercharging pressure control system, in spite of the drop in the rotational speed of the engine, which makes it difficult to carry out stable control of the supercharging pressure.
Still further, in general, when the engine is accelerated with the transmission in a lower speed position (e.g. the first speed position), the rate of rise of the engine rotational speed is larger than when the engine is accelerated with the transmission in a higher speed position. Accordingly, the rate of increase of the supercharging pressure is higher in the former case. This is conspicuous especially at the sudden standing-start of the vehicle. However, according to the aforesaid conventional method, the supercharging pressure is controlled in the transient condition, always in the same manner, irrespective of the gear position of the transmission. Further, the control system has an inherent time lag in its responsiveness. Consequently, when the supercharging pressure is in the transient condition with the transmission in a lower speed position, the rate of increase of the supercharging pressure exceeds the control speed of the system, so that the engine output is suddenly increased, resulting in spinning of the driving wheels of the vehicle and overboosting. Hence, good accelerability of the engine cannot be obtained.
Also, according to the conventional method, the desired supercharging pressure, at which the supercharging pressure is to be controlled in feedback control mode during the steady condition, is set at a single constant value, irrespective of the gear position of the transmission. As a result, the torque acting upon the transmission component parts will become large when the transmission is in the lower speed position during the steady condition of the supercharging pressure, which can cause overboosting and can badly affect the durability or life of the engine.
Further, it is desirable to stop supercharging the engine when the engine is in particular operating conditions, such as a condition in which the intake air temperature or the cooling water temperature is very low or very high, and a condition in which the supercharging pressure is very high. However, if the supercharging pressure is increased immediately after the engine has left such particular conditions, alternate supercharging and interruption thereof can be repeated at the boundary between such particular operating conditions and other operating conditions adjacent thereto, which renders the supercharging pressure unstable and can even badly affect the durability of the engine.
Furthermore, if the supercharging pressure is increased at the start of the engine in cold weather, where the engine operation is unstable, it will cause abnormal combustion within the combustion chamber due to the increased charging efficiency. Therefore, conventionally, the supercharging pressure is decreased at the start of the engine in cold weather and before the engine is warmed up. However, the predetermined temperature, for ascertaining whether or not the engine has been warmed up, is set at a relatively low value corresponding to the temperature of the engine before being warmed up. Therefore, the supercharging pressure starts to be increased before completion of the warming-up of the engine when the engine temperature exceeds the predetermined temperature, and thereafter it is further increased with an increase in the engine rotational speed. As a result, the engine can be brought into a high load condition before being warmed up, also adversely affecting the durability of the engine.
Also, in an internal combustion engine with a supercharger in general, an intercooler is arranged in the intake pipe downstream of the supercharger. The cooling effect of the intercooler varies depending upon running conditions of the vehicle. For example, when the ambient air temperature is low, or when the vehicle is running at a high speed, the cooling effect of the intercooler increases so that the temperature of intake air supplied to the engine becomes too low, which results in an excessive increase in the charging efficiency of the intake air and hence an overload on the engine. This also adversely affects the durability of the engine,
To eliminate this disadvantage, it has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-128930, to decrease the supercharging pressure by a predetermined amount when the intake air temperature is extremely low.
However, in actuality, even when the intake air temperature is low, the engine is not overloaded if its rotational speed is low. On the contrary, if the supercharging pressure is decreased irrespective of the engine rotational speed, merely on the condition that the intake air temperature is low, the supercharging pressure will slowly increase at the start of the engine, resulting in an insufficient supercharging effect.